Conventional surface plasmon resonance (SPR) sensors can detect chemical and biological events through spectroscopy of surface plasmons. Surface plasmons are collective oscillations of conduction electrons, which can exist at the physical interface of metal and dielectric media. Surface plasmons can be excited by optical waves. Resonant coupling occurs between the optical wave and the surface plasmons and results in a decrease in intensity of the optical wave. In the attenuated total reflectance method, an optical wave is introduced into an optical prism having a thin metal layer deposited on a base of the prism. When introduced to the base, light evanescently tunnels though the metal film and excites a surface plasmon at the outer boundary of the metal film. Alternatively, an optical wave can be coupled to surface plasmons via diffraction on a meta-coated diffraction grating. When the incident optical wave is polychromatic, the coupling condition is only fulfilled for a narrow band of wavelengths and the spectrum of the optical wave coupled to a surface plasmon contains a characteristic spectral feature, typically a dip. Spectral position of the SPR dip is sensitive to changes in the refractive index of the medium in which the surface plasmon propagates. Therefore, very small changes in the refractive index at the surface of the metal film can be measured by monitoring the position of the SPR dip. The binding of analyte molecules to biorecognition elements immobilized on the surface of the metal film creates a shift in the spectral position of SPR dip-resonant wavelength. The change in the resonance wavelength can be correlated with the concentration of analyte.
SPR sensors are used across many fields. In food quality and safety, SPR sensors have been used to detect bacteria, such as Escherichia coli and Salmonella enteritidis. SPR sensors can also detect drug residues, hormones, allergens, proteins, chemical contaminants, and toxins produced by bacteria. For medical diagnostics, antibodies, drugs, hormones, and disease biomarkers, including cancer, allergy, and heart attack markers can be identified with SPR to corroborate normal biological and pathogenic processes, or pharmacologic responses to therapeutic intervention. SPR sensors are also used to detect the presence of pesticides and heavy metals in the environment.
Conventional SPR sensors include prism couplers, diffraction gratings, optical fibers, and integrated optical waveguides. These SPR sensors are expensive, large, and often maintained in a centralized laboratory to facilitate sharing. Monitoring immediate changes in environmental or health related conditions are generally impracticable. Moreover, conventional SPR sensors are generally inadequately sized for immediate and mobile use, and cost disfavors widespread adoption.